This invention relates to plastic molding processes and more particularly to the molding of plastic encapsulated semiconductor devices.
In a conventional method of manufacturing semiconductor devices, and particularly, integrated circuits, the semiconductor chip or die is mounted on a lead frame. The lead frame has a support pad for the die and a plurality of fingers that provide the electrical terminals for the device. Following suitable bonding between portions of the semiconductor device and the fingers of the lead frame, the lead frame and the semiconductor chip is placed in a molding die which is closed there-around and molding compound injected therein to form the molded plastic package. The tie-bar portions of the lead frame are then removed to form separated electrical terminals for the finished device.
Whatever polymer utilized in the foregoing process, epoxy, phenolic or silicone, the process, like all transfer molding operations, results in highly cross-linked material that sticks to the mold surfaces, resulting in considerable down time for cleaning and renewing the mold surfaces. Cleaning is generally accomplished either by vigorous mechanical brushing or molding with a compound more reactive and stronger than the molding compound already adhered to the surfaces. In the latter case of molding with the cleaning compound, reaction of that compound with the contaminating surface occurs to an extent that when the molded cleaning compound is removed from the mold, the contaminating material, reacted with and sticking to the cleaning compound, is also removed. In that particular case, melamine is the reactive cleaning compound having the generic structure ##STR1## where hydrogen may be partly replaced by alkyl groups of any size or shape. As is disclosed in the co-pending application, above-referenced, such a cleaning compound is substantially improved by the addition of N-methyl-2-pyrrolidone.